1、He's been drinking whiskey on the sly for thirty years.
他暗中偷喝威士忌葡萄酒已有三十年了。

2、He drives his mother's car on the sly while she's out at work.
在他母親出去工作時，他偷偷開了她的汽車。

3、I believe that Malcolm is meeting his secretary on the sly after work.
我敢確實馬爾科姆下班后要和他的秘書幽會。

on the sly翻譯